3.6.2 - Nervous Coordination Flashcards
Explain how a resting potential is maintained across the axon membrane in a neurone.
- potassium ions diffuse out/sodium ions diffuse in
- membrane more permeable to potassium ions leaving than sodium ions entering
- sodium ions actively transported out and potassium ions in
Explain why the speed of transmission of impulses is faster along a myelinated axon than along a non-myelinated axon.
- myelination provides electrical insulation
- saltatory conduction/depolarisation at nodes of Ranvier in myelinated
- depolarisation occurs along whole/length of axon in non-myelinated
Describe the sequence of events involved in transmission across a cholinergic synapse.
- depolarisation of presynaptic membrane
- calcium channels open and calcium ions enter synaptic knob
- calcium ions cause synaptic vesicles to move/fuse with presynaptic membrane and release acetylcholine (neurotransmitter)
- acetylcholine (neurotransmitter) diffuses across synaptic cleft
- acetylcholine attaches to receptors on the postsynaptic membrane
- sodium ions enter postsynaptic neurone leading to depolarisation
Give two reasons why transmission across a cholinergic synapse is unidirectional.
- only the presynaptic neurone/knob/membrane releases/has
neurotransmitter/acetylcholine - only the postsynaptic neurone/membrane has receptors/no receptors in the presynaptic neurone/membrane
nerve impulse
- a self-propagating wave of electrical activity that travels along the axon membrane
- temporary reversal of potential difference across the axon membrane
resting potential
- inside of axon is negatively charged relative to the outside
- approx. -70mV
- axon membrane said to be polarised
how is a resting potential established?
- 3 sodium ions are actively transported OUT of the axon by the sodium-potassium pumps
- 2 potassium ions are actively transported INTO the axon by the sodium-potassium pumps
- outward movement of sodium ions > inward movement of potassium ions
- electrochemical gradient created
- sodium ions diffuse into axon
- potassium ions diffuse out of axon
- membrane more permeable to potassium ions
action potential
- sufficient size stimulus
- negative charge of -70mV becomes a positive charge of approx. +40mV
- axon membrane said to be depolarised
how is an action potential generated?
- resting potential: some potassium channels (permanently) open but sodium channels are closed
- stimulus causes some sodium channels to open → sodium ions diffuse into axon → reversal in potential difference (depolarisation)
- more sodium channels open
- action potential (+40mV) established
- sodium channels close and potassium channels begin to open
- potassium ions diffuse out and more potassium ion channels open (repolarisation)
- temporary overshoot → inside of axon becomes more negative than usual (hyperpolarisation)
- potassium channels close and resting potential is re-established by the sodium-potassium pump (repolarisation)
passage of an action potential along a myelinated axon
- myelin sheath acts as electrical insulator → prevents action potentials from forming
- nodes of Ranvier are breaks in myelin sheath where action potentials can occur
- saltatory conduction → action potentials jump from node to node
- faster passage than in unmyelinated
how does diameter of axon affect action potential speed?
- greater diameter of axon = faster speed of conductance
- less leakage of ions from large axon
- leakage makes membrane potentials harder to maintain
how does temperature affect action potential speed?
- higher temperature = faster nerve impulse
- affects rate of diffusion of ions
- energy for active transport comes from respiration → enzyme-controlled
- optimum enzyme function at higher temperatures (but not too high)
all-or-nothing principle
all: any stimulus above the threshold value will succeed in generating an action potential and so a nerve impulse will travel
nothing: any stimulus below the threshold value will fail to generate an action potential
refractory period
- after an action potential has been generated
- sodium channels are closed
- inward movement of sodium ions is prevented
- impossible for a further action potential to be generated
purposes of refractory period
- ensures that action potentials are propagated in one direction only → can’t be propagated in a region that is refractory so only move in forward direction rather than spreading out in both directions
- produces discrete impulses → refractory period means that new action potential can’t be formed immediately behind the first one, keeping them separate
- limits the number of action potentials → as they are separated, a limited number can pass along an axon in a given time
unidirectionality of synapses
information can only be passed in one direction, from the presynaptic neurone to the postsynaptic neurone
summation at synapses
spatial: several presynaptic neurones together release enough neurotransmitter to exceed threshold value of postsynaptic neurone, triggering a new action potential
temporal: a single presynaptic neurone releases neurotransmitter many times over a very short period (action potential triggered if threshold value is exceeded)
inhibitory synapses
- neurotransmitter released from presynaptic neurone binds to chloride ion protein channels on postsynaptic neurone, which open
- chloride ions enter postsynaptic neurone by facilitated diffusion
- binding of neurotransmitter causes opening of nearby potassium protein channels
- potassium ions move out of the postsynaptic neurone into the synapse
- inside of postsynaptic membrane becomes more negative and outside becomes more positive
- membrane potential increases to as much as -80mV (hyperpolarisation)
- makes it less likely that a new action potential will be created as a larger influx of sodium ions is needed to produce one
excitatory synapses
synapses that produce new action potentials
cholinergic synapse
- action potential arrives at end of presynaptic neurone
- calcium ion protein channels open
- calcium ions enter synaptic knob by facilitated diffusion
- influx of calcium ions causes synaptic vesicles to fuse with presynaptic membrane
- acetylcholine (neurotransmitter) released into synaptic cleft
- acetylcholine diffuses across cleft and binds to receptor sites on sodium ion protein channels in membrane of postsynaptic neurone
- sodium ion channels open and sodium ions diffuse in rapidly along a concentration gradient
- influx of sodium ions generates new action potential in postsynaptic neurone
- acetylcholinesterase hydrolyses acetylcholine into acetyl (ethanoic acid) and choline which diffuse back across cleft into presynaptic neurone
- recycling
- rapid breakdown of acetylcholine also prevents continuous generation of new action potentials in postsynaptic neurone
- therefore produces discrete impulses
- ATP released by mitochondria used to recombine choline and acetyl (ethanoic acid) into acetylcholine
- stored in synaptic vesicles for future use
- sodium ion protein channels close
neuromuscular junction
- synaptic vesicles fuse with presynaptic membrane and release their acetylcholine
- acetylcholine diffuses to postsynaptic membrane (of muscle fibre)
- alters permeability to sodium ions which enter rapidly and depolarise the membrane
- recycling occurs
neuromuscular junction vs cholinergic synapse
both:
- have neurotransmitters that are transported by diffusion
- have receptors that cause an influx of sodium ions
- use a sodium-potassium pump to repolarise the axon
- use enzymes to break down neurotransmitter
neuromuscular junction:
- excitatory
- links neurones to muscles
- involves motor neurones
- action potential ends here
- acetylcholine binds to receptors on muscle fibre membrane
cholinergic synapse:
- excitatory or inhibitory
- links neurones to neurones or effectors
- involves motor, sensory and relay neurones
- new action potential may be produced along another neurone
- acetylcholine binds to receptors on membrane of postsynaptic neurone
